1. Field of the Invention
The present invention relates to a power control circuit of an on-vehicle electronic controller and, particularly, a power control circuit composed of a switching regulator and a series regulator.
2. Description of the Related Art
A power control circuit that is constituted of a switching regulator or a series regulator has been known, and disclosed in patents, such as JP-A-9-37545. In such a power control circuit, voltage of an external power source is reduced to supply stable electric power at a desired voltage.
The switching regulator includes a switching transistor which is connected in series with a power source, and switches the switching transistor on and off to supply necessary power to a smoothing circuit. This switching regulator can operate at low and favorable power consumption but at low and unfavorable control accuracy.
On the other hand, the series regulator includes a power transistor which is connected in series with a power source to control the conductivity of the power transistor so that the transistor output voltage to be supplied to a smoothing circuit can be regulated. This series regulator can operate at high and favorable accuracy but at high and unfavorable power consumption.
JP-A-6-335238 discloses a composite control circuit in which the switching regulator and the series regulator are connected in series. The switching regulator reduces voltage of an external power source to a medium voltage that is slightly higher than a desired voltage at low power consumption, and the series regulator reduces the medium voltage to a desired voltage at high accuracy.
However, if an excessive amount of current or overcurrent flows in the series regulator by accident, the switching regulator of the above composite control circuit may not stably operate without a protection circuit, which is expensive and bulky.